The optical communication system of the related art comprises a transmitting terminal for generating a WDM optical signal formed by wavelength-division multiplexing of a plurality of optical signals of different wavelengths, an optical transmission line for transmitting the WDM optical signal transmitted from the transmitting terminal and a receiving terminal for receiving the transmitted WDM optical signal. Moreover, this optical communication system comprises, as required, one or a plurality of optical repeaters having the function to amplify the WDM optical signal in the course of the optical transmission line.
In such an optical communication system, the waveform of each optical signal is deteriorated due to non-linear optical effects in the optical transmission line. In order to eliminate the deterioration of the waveform, it is effective to reduce the optical power of the optical signals launched into the transmission line, but a reduction of the optical power results in an increase of the optical signal to noise ratio (OSNR) due to noise accumulation in the optical amplifiers.
For this purpose, it has been proposed to use a combination of discrete optical amplifiers provided within repeaters and distributed optical amplifier using the optical transmission line in common as the optical amplifying medium. In a discrete optical amplifier the amplifying medium and pump light source are centralized in one area. In contrast, the amplifying medium of a distributed optical amplifier is laid between two remote places and pump light source is provided in one or both places.
Fiber doped optical amplifiers represent one group of optical fiber amplifiers. In doped fiber amplifiers a lanthanide rare-earth element is added to the optical fiber. The structure of electronic excitation levels of lanthanide rare-earth atoms allows for amplification by stimulated emission in the low-absorption wavelength domain of optical fibers. The operation bandwidth is limited to certain wavelength ranges: Neodymium (Nd) amplifies in the 1060 nm wavelength band, Praseodymium (Pr) in the 1300 nm wavelength band, Thulium (Tm) in the 1450 nm wavelength band and Erbium (Er) in the 1550 nm band.
The other group of optical fiber amplifier takes advantage of stimulated Raman scattering (SRS) an inelastic scattering process between photons and optical phonons of lattice vibrations. It has a wide gain width and a gain shift of 13.3 THz (about 100 nm), as will be described later with reference to FIG. 4. In contrast to erbium doped fiber amplifiers, the SRS effect occurs also in ordinary optical fibers. Moreover, the pumping wavelength can be set for any amplification wavelength.
The low loss transmission window in silica-based optical fibers covers the wavelength range from 1450-1650 nm with a minimum around 1550 nm. Until recently, only Erbium doped fiber amplifiers (EDFA) which cover the so-called C-band (1530-1565 nm) and the gain-shifted EDFA which cover the so-called L-band (1570-1605 nm) were employed. In these systems the pump wavelengths for distributed Raman amplification (DRA) are much shorter than the signal wavelengths.
The increasing demand for transmission capacity of optical fiber systems requires the expansion of the optical bandwidth in a single fiber. Extension to longer wavelengths has several drawbacks. The loss profile in this wavelength domain varies strongly among installed fibers, which makes system design more difficult and materials and technologies for optical components (e.g. photodiodes) yet have to be developed. Raman amplification is in principle available for this wavelength domain. However, the pump wavelengths would partly overlap with the short wavelengths signals in the C-band.
On the short wavelength side below 1530 nm, the low loss region of silica-based fibers extends to 1450 nm. Raman pump wavelengths for this region do not overlap with signals; however, they are located at the water-peak of optical fibers, where the absorption loss is high. Nevertheless, due to the availability of high-power pump lasers, Raman amplification is a feasible technology for this wavelength domain. Besides, Thulium doped amplifiers and gain-shifted Thulium doped amplifiers are candidates as amplifiers for wavelength bands below 1530 nm. The additional wavelength regions are referred to as S+ band (1450-1490 nm) and S band (1490-1530 nm). In these new wide-bandwidth systems, the short wavelength signals act as DRA pump light with respect to the long wavelength signals. S+ and S band wavelengths transfer optical power to the C and L band channels via SRS. Distributed Raman pumping of the S+ and S band channels compensates the power depletion due to SRS as well as the increased fiber loss at S+ and S wavelengths. If all wavelengths are in service, the power transfer is balanced.
A further description will be given of the conventional optical communication system with reference to the accompanying drawings.
FIG. 1 is a graph of a typical optical loss spectrum of silica-based optical fibers in which the low loss region covers the wavelength range from 1450 to 1650 nm. Optical amplifiers allow simultaneous amplification of a group of wavelengths. The C and L band correspond to the wavelength ranges of Erbium doped and the gain-shifted Erbium doped amplifiers. The S+ and S band are related to the wavelength ranges of Thulium doped and gain-shifted Thulium doped fiber amplifiers. When light of 1450 nm and 1550 nm travels 100 km through an optical fiber with a loss of 0.26 dB/km, it experiences a loss of 26 dB and 20 dB, respectively. Thus, light with a wavelength of 1450 nm experiences a loss of about 0.06 dB/km higher than the lowest loss wavelength.
FIG. 2A shows a conventional WDM transmission system. Symbols of optical components in the accompanying drawings including FIG. 2A are defined as shown in FIGS. 3A through 3F. FIG. 3A shows various types of optical amplifiers. The C and L band can be amplified either separately by means of a broadband C/L band amplifier. Accordingly, S and S+ bands can be amplified either by separate doped fiber amplifiers or Raman amplifiers, or by amplifiers covering the whole S+ and S band wavelength range. For this group of amplifiers a double lined triangle is used in this specification. Variable optical attenuators (VOA) can be added to the amplifiers as means for adjusting the amplifier output power.
FIG. 3B shows an optical circulator, and FIG. 3C shows an optical tap. FIG. 3D shows an optical switch. FIGS. 3E and 3F show a WDM coupler.
Turning to FIG. 2A again, the WDM transmission system includes a transmitter, a transmission fiber connecting remote locations, discrete optical amplifier to compensate for the fiber loss, and a receiver. Multiple wavelengths transmission enhances the transmission capacity. The optical amplifiers add noise in the form of amplified spontaneous emission, which reduces the optical signal-to-noise ratio, thus giving rise to errors in the signal detection. Distributed Raman amplification can improve the signal to noise ratio because it amplifies the signals along the transmission fiber. Moreover, the stimulated Raman scattering tilt, which will be described later in detail, can be compensated for in the system. There are control schemes that allow adjusting the spectral tilt under changing conditions of C and L band channel usage (OECC'99, “Optical SNR degradation due to stimulated Raman scattering in dual-band WDM transmission systems and its compensation by optical level management”, T. Hoshida, T. Terahara, J. Kumasako and H. Onaka).
Distributed Raman amplification generally is not high enough to make discrete amplifiers obsolete. As shown in FIG. 2B, counter-propagating amplification is used to average out bit-pattern dependent amplification causing power fluctuations. Commercial systems employ C and L band amplifiers. In the laboratory triple band (S, C, L) transmission has been demonstrated (ECOC2000, “Experimental Study on SRS loss and its compensation in three-band WDM transmission”, Yutaka Yano, Tadashi Kasamatsu, Yoshitaka Yokoyama and Takashi Ono), as shown in FIG. 2C.
In dense WDM systems, channel-interleaved bi-directional transmission as shown in FIG. 2D can reduce impairments due to nonlinear interaction between adjacent channels (cross phase modulation, four wave mixing) and thus allows increasing the spectral efficiency of the system. At the amplifier stage, optical circulators (directional coupling elements) separate forward and backward propagating channels.
FIG. 4 shows the optical power depletion due to stimulated Raman scattering and fiber loss. In wide-band WDM transmission systems with high channel count, SRS causes a strong power transfer from the short wavelengths to long wavelengths. The Raman gain depends on the frequency shift between the shorter and the longer wavelength. It has a maximum around 13.3 THz in silica-based fibers. Thus, for distributed Raman pumping it is most effective to allocate the pump wavelength shifted about 100 nm to shorter wavelength with respect to the signal wavelengths. In wide band WDM systems the short wavelength signals become efficient pump light sources for the long wavelength channels.
FIG. 5 shows SRS-spectral tilt compensation using DRA and pre-emphasis (repeater output level control). Using pre-emphasis and distributed Raman amplification of the short wavelength channels, the higher absorption loss and the SRS power depletion can be compensated (ECOC2000, “Experimental Study on SRS loss and its compensation in three-band WDM transmission”, Yutaka Yano, Tadashi Kasamatsu, Yoshitaka Yokoyama and Takashi Ono). It is to be noted that the power transfer is balanced if all channels are on.
However, in wide-bandwidth systems, an interruption of the operation of the short wavelength channels (either by failure or for the purpose of maintenance) or a reduced number of active short wavelength channels result in less or no power transfer to the C and L band signals. As a consequence, the C and L band signal output power drops and the OSNR degrades, making these channels more error-prone.
Thus, a general object of the present invention is to provide a control scheme for long wavelength channels in wideband WDM optical fiber transmission system in which the above problem is overcome.
A more specific object of the present invention is to provide an optical amplifier device capable of protecting long wavelength channels of wideband optical fiber transmission systems in which the power transfer from the short to the long signal wavelengths due to stimulated Raman scattering is essential for the transmission of the long wavelength signals.
Another object of the present invention is to provide an optical communication system utilizing the above protection scheme.